## Pathophysiology and Diagnostic Features of Myasthenia Gravis **Key Point:** Edrophonium (Tensilon) is a short-acting anticholinesterase used diagnostically, NOT therapeutically. Its effect lasts only 5–10 minutes, not hours. ### Mechanism of AChR Antibody Pathology Anti-AChR antibodies cause neuromuscular junction dysfunction through three mechanisms: 1. **Complement-mediated destruction** — IgG antibodies bind AChR and activate the classical complement cascade, leading to membrane attack complex (MAC) formation and postsynaptic membrane damage. 2. **Cross-linking and internalization** — Antibodies cross-link AChR molecules, accelerating their internalization and degradation. 3. **Functional blockade** — Antibodies sterically block acetylcholine binding. **High-Yield:** The complement-mediated mechanism is the PRIMARY pathogenic pathway and is why corticosteroids and immunosuppressants are effective. ### Diagnostic Testing | Test | Findings in MG | Duration/Notes | |------|---|---| | Anti-AChR antibodies | Positive in ~85% of generalized MG | Diagnostic; high specificity | | Repetitive nerve stimulation (RNS) | Decremental response (>10% amplitude decline) | Positive in ~60–70% seropositive; less sensitive in seronegative | | Edrophonium test | Brief improvement (5–10 min) in strength | **Diagnostic only**; short-acting; rarely used now | | Single-fiber EMG | Increased jitter, blocking | Most sensitive (~95%); not specific | **Clinical Pearl:** Edrophonium is a diagnostic agent—it temporarily blocks acetylcholinesterase, allowing accumulated acetylcholine to overcome the AChR deficit. The effect is transient and NOT sustained. ### Thymectomy Indications **Key Point:** Thymectomy is recommended in: - Seropositive generalized MG (standard of care) - Seronegative generalized MG (benefit shown in clinical trials; recommended) - Thymoma-associated MG (mandatory) Thymectomy is NOT recommended in purely ocular MG without systemic features. **High-Yield:** Even seronegative patients with generalized MG benefit from thymectomy, as thymic abnormalities (hyperplasia, thymoma) are present in ~85% of MG patients. ### Summary Table: Correct vs. Incorrect Statements | Statement | Correct? | Reason | |---|---|---| | AChR antibodies → complement-mediated destruction | ✓ Yes | Primary mechanism of NMJ damage | | Edrophonium produces sustained improvement | ✗ **NO** | Effect lasts only 5–10 minutes; diagnostic only | | Thymectomy recommended in seronegative generalized MG | ✓ Yes | Evidence-based; thymic abnormalities present in most | | RNS shows decremental response in seropositive MG | ✓ Yes | Positive in ~60–70% of seropositive patients | **Warning:** Do not confuse edrophonium (diagnostic, transient) with long-acting anticholinesterases like pyridostigmine (therapeutic, sustained effect over hours).
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